You have several Alternatives:
a) On the Root of your Site Create or Edit .htaccess file Inserting:
DirectoryIndex homepage.phpIf you are in a shared Hosting this will be the only Feasible alternative.
b) Open Apache httpd.conf and in the <IfModule mod_dir.c> section Insert:
DirectoryIndex homepage.php
c) Set up mod_rewrite to redirect index.php to homepage.php
(If you are Working on Localhost After you will need to Restart the Apache Server...)
The DirectoryIndex Apache directive simply give an Ordered List of the Default Addresses, so if you want you can include Multiple Possibilities like for Example:
DirectoriIndex homepage.php homepage.html index.php index.html
Apache will try to Find the First Existing one and after will Display that.